In chemical or biological processes such as waste treatment and Flue Gas Desulphurization, it is common for these processes to be carried out in a stirred vessel where gas, such as oxygen or an oxygen containing gas, is injected into the agitated liquid through a submerged agitator. These aforementioned processes are oftentimes utilized by municipalities and industry to treat waste and to produce chemicals and chemical byproducts, wherein the process requires the gas to be introduced to the liquid simultaneously as it is being agitated. The gas is commonly compressed and/or pressurized gas injected into the agitated liquid through the agitator drive and agitator via a sparge arrangement.
Current methods and apparatuses for the injection of gas into an agitated liquid via a submerged agitator require special construction, including specialized parts and components that tend to increase manufacturing and operating costs while decreasing agitator reliability.
For example, conventional methods for injecting gas into an agitated liquid involve passing compressed gas through the drive components of the agitator, including the gear drive, the agitator or mixer shaft, and the shaft support bearings and/or flexible couplings.
Current agitator drive/gas injection technology typically includes a large compressor connected to a rotary joint of the agitator drive via stationary piping. The compressor generates compressed gas or air which is usually high in temperature due to the compression, enabling it to overcome the static head pressure of the liquid column in the agitator shaft and travel to the delivery points of the sparge arrangement located on the submerged agitator. The rotary joint is typically connected to the gear drive of the agitator through a specialized connection. Because present methods of injecting gas into a liquid entail delivery through the agitator shaft, the gear drive of the agitator must have a large shaft bore. This is required to allow sufficient gas flow at minimum pressure drop as well as providing adequate spacing for placement of insulation and a corrosion resistant liner. The insulation and liner function to promote adequate service life by keeping the bearing and the gear drive lubricants cool.
Current gas injection apparatuses also include flexible couplings that connect the agitator shaft to the gear drive along with specialized bearings for supporting the shaft which require additional mounting considerations such as hydraulic nuts that increase the cost of manufacture. The specialized bearings must accommodate varying bearing clearances and tolerances due to shaft temperature change. In addition, current gas injection apparatuses require use of a shaft seal to prevent gas in the vapor space of the mixing vessel from escaping into the atmosphere.
The aforementioned special construction of the current agitator drive/gas injection apparatuses is due in part to the elevated temperature of the compressed gas, which often times reaches temperatures of 250° F. or higher. At these elevated temperatures, commonly used lubricants for gear drives, flexible couplings and bearings breakdown more rapidly, requiring increased maintenance to prevent premature failure.
For this reason, the components of current gas injection apparatuses are oversized and the apparatuses are specially configured and use special lubricants in the seals and couplings along with using insulation and corrosion liners to accommodate the high gas temperatures. Furthermore, the drive gear bearings must be oversized, require special lubrication, maintenance intervals and specialized assembly settings to accommodate the varying operating clearances between the bearings and the drive shaft, that result from the elevated gas temperature.
The present apparatuses also attempt to address the elevated gas temperatures by cooling the gas prior to delivering it to the agitator. The gas can be cooled by natural convection from the distribution piping, it may be cooled by a specialized cooler. The specialized cooler, however, oftentimes requires significant outlays in terms of manufacturing cost and operating cost.
Accordingly, it is desirable to provide a method and apparatus for effectuating improved gas injection through an agitator drive which provides improved agitator reliability while reducing the need for specialized components and construction and therefore reducing manufacturing and operating costs.